Another aspect of the problem of ghosts is related to the stability
of classical solutions.
The most important issue related to massive ghosts is whether
their presence can be compatible with the stability of the classical
solutions of GR. As we mentioned above, in higher derivative models
of gravity one typically meets the Planck suppression. As a result,
classical solutions of GR represent high-quality approximations to
the corresponding solutions in the presence of the higher-derivative
terms. This logic can be successfully applied to the fourth-derivative
theory and extended to the polynomial theories (REF ), if
we assume that all massive parameters are of the Planck order
of magnitude [1]}.
